I've been out of the dive travel circuit for a few years, but from 1995 to 1999, my wife and I traveled to the Caribbean once a year -- Coz, St. John/St. Thomas, St. Lucia, Paradise island, Puerto Rico. For cheap, I'd recommend Coz. Outstanding, inexpensive, and experienced dive operators right at or next to most hotels. Most enjoyable diving I've ever had. I wouldn't bother w/ all-inclusives as beverages and food are cheap in town which is very short cab ride away. Hotels are more expensive. US$ goes a long way in Mexico, but nite life was somewhat limited(as compared to Cancun).
The Saints were all more expensive, but still not insane. At the time, St. Lucia seemed to cater to non-American vacationers -- met folks from Scotland, Germany, Canada and South Africa. Lucia was also a little rough around the edges. St. John zero nitelife due to Nat'l Park. St. Thomas is a little spread out - we rented a car.
PR -- did too much drinking and not enough diving
PI -- nitelife is centered around the Atlantis complex, diving was convenient and varied(wrecks, sharks and a few hole dives), very safe, gambling, eating etc.
My info may be somewhat dated, but things probably haven't changed too much. Hopefully, it will help you out. Have fun and if you go to Mexico have a few of the grande margaritas for me.
LobstaMan